A sector forming a part of a camera shutter is generally a mechanical element which is actuated by rotating a motor to define an exposure quantity and an aperture diameter. As a consequence, if the rotation of the motor fluctuates due to a change in battery power, the exposure quantity and aperture diameter cannot be defined properly, and synchronized operation of a strobe light cannot be attained, thereby resulting in inferior photography.
According to Japanese Laid-Open Patent Document No. 61-163027 "Speed Control Device of Camera Shutter Driving Motor", the shutter closing time relating to an exposure quantity E.sub.V is stored in a memory circuit in the form of data for the control of motor speed or data for the control of the width of driving pulses to be sent to a motor. As soon as the operating speed of a sector is detected, the width of the driving pulses is determined as a function of the data stored in the memory circuit, so that proper exposure is attained by controlling the rotational speed of the motor.
According to Japanese Laid-Open Patent Document No. 62-194239 "Camera Shutter Control Device", the time from the starting of rotation of a shutter driving motor to the time that a sector begins to open is detected, and in response to the time thus detected, the closing time of a shutter opening is set, so that a proper exposure is attained irrespective of battery voltage.
According to Japanese Laid-Open Patent Document No. 16-163027 and Japanese Laid-Open Patent Document No. 62-194239 as described above, in order to prevent the motor from operating improperly due to fluctuation in battery voltage, the rotational speed of the motor is controlled, or one data element from an exposure control data table is selected corresponding to brightness, as a function of the motor speed.
When the shutter sector is actuated by a motor whose rotor comprises a permanent magnet, however, the relationship between the opening diameter and the elapsed time changes due to several parameters, one of which is the speed of the sector.
Therefore, if the sector speed and the brightness are subjected to calculation and the calculation result is used to access the data, accurate control cannot be attained.
Further, if accurate calculation is desired, an arithmetic circuit for making the calculation must be complicated, and the volume of required data increases. It is therefore hard to control the actuation of the sector in a short time.